The present invention relates to a one-piece f.THETA.-DEC scanning lens, particularly to a f.THETA.-DEC scanning lens comprising a one piece lens instead of a conventional two piece lens, which is capable of compensating the Wobble effect, scanning linearly, and decreasing setup volume at the same time.
Referring to FIG. 1a, FIG. 1a is a schematic diagram which shows a conventional two-element f.THETA. lens group used in laser printer. In FIG. 1a, a parallel light beam, emitted by a light source and a collimating lens 10, passes through a cylindrical lense 20 having curvature in the direction of the secondary scanning plane (the secondary scanning plane is defined as the plane perpendicular to the scanning plane), so that the light beam is focused on the polygon mirror scanner 30. When the polygon mirror scanner 30 rotates, the light beam is reflected by the polygon mirror 30 so as to form a scanning light beam at the scanning plane. Furthermore, the scanning light beam passes through a two-element f.THETA. lens group 40 which is composed of a meniscus lens 42 and a cylindrical lens 44 so that the scanning light beam is capable of being projected on an imaging surface (for example, a light-sensing drum 50), while the two-element f.THETA. lens 40 has to have the following effects:
(1) causing the light beam to be focused onto a straight line on the imaging surface;
(2) because the polygon mirror scanner rotates at a uniform angular velocity, the two-element f.THETA. lens has to convert the incident light beam rotating the same as the polygon mirror scanner into the light beam moving linearly on the imaging surface;
(3) as shown in FIG. 1b, compensating the Wobble effect resulting from the wobbling of the polygon mirror scanner 30.
However, if the optical scanning system is composed of all spherical lenses, then in the secondary plane, because the light source emits a light beam that passes through a cylindrical lens before polygon mirror scanner so that it can't form an optical conjugate relationship between object and the imaging surface, the Wobble effect can not be compensated. In addition, because there is few free variables, the scanning linearity of the light beam can't reach an order below 0.5% within the scope of .+-.105 mm (the width of A4 paper), and also the field curvature at the image side is very large. This kind of f.THETA. lens groups can not be used for high resolution laser printer.
As shown in FIG. 1a, a cylindrical lens is used for the purpose of improving the defects of the spherical lens for which the Wobble effect can not be calibrated. However, due to the rather large field curvature induced by the cylindrical lens on the secondary scanning plane, the cylindrical lens can be only placed near the image surface. Consequently, it requires a lot of space, and increases manufacturing costs at the same time.